Thanks alexis–much appreciated.
As far as your question… You COULD route a hardware processor’s returns to an external input, but I have hard time trying to figure out why you’d want to do it, or what advantage would be gained. The primary purpose of external inputs is to give you a way to route things to your control room (i.e. to your monitoring environment) so that they aren’t a part of the main mix environment–allowing you to listen to them without affecting anything about your mix. There are several applications for external inputs in this regard. Although external inputs can ALSO be routed through the Cubase mixer by setting them up as an input to an audio track, you’d probably need to have a really special reason to route the returns of an external processor unit to an external input. Never say never, but I honestly can’t think of a reason why you’d want to do that.
If you are indeed using your piece of hardware as an external processing device, I would set it up under “external FX” in VST connections, and then just use it as a traditional insert effect in the mixer (you can even use it in the control room inserts if you want).